Recently, aluminum foil has generally been used as a current collector for positive electrodes of a lithium-ion battery and an electrical double layer capacitor. In addition, such a battery and a capacitor have been used in electrical vehicles and the like in recent years, and the electrode with the current collector in the battery and the capacitor has been required to have a higher output and a higher energy density with the broadening of the usage purposes. As described in Patent Documents 1 and 2, porous aluminum body which includes open pores having a three-dimensional network structure has been known as a current collector for an electrode.
As a method for producing this porous aluminum body, a foam-melting method has been known as disclosed in Patent Document 3. In this foam-melting method, a thickener is added to a melted aluminum so as to increase the viscosity, and then titanium hydride as a foaming agent is added thereto. While foaming the melted aluminum by utilizing hydrogen gas generated in a thermal decomposition reaction of the titanium hydride, the melted aluminum is solidified. However, foamed aluminum obtained by this method includes large closed pores having sizes of several millimeters.
There are other methods, and the following method is exemplified as a second method. Aluminum is pressed into a casting mold having a core of sponge urethane, and a hollow cavity formed after burning off the urethane is filled with the aluminum. Thereby, foamed aluminum having a sponge skeleton is obtained. According to this method, foamed aluminum is obtained which includes open pores having pore diameters that fulfill 40 PPI or smaller, that is, 40 cells or less per inch (pore diameters of about 600 μm or larger).
The following method is exemplified as a third method. As disclosed in Patent Document 4, aluminum alloy is infiltrated into a reinforcing material made of hollow ceramics by the pressure infiltration method; and thereby, foamed aluminum is obtained which includes closed pores having pore diameters of 500 μm or smaller in accordance with the dimension of the reinforcing material.
The following method is exemplified as a fourth method. As disclosed in Patent Document 5, a mixed powder of AlSi alloy powder and TiH2 powder is sandwiched between aluminum plate materials, and the mixed powder is heated and rolled in such a state. Thereby, aluminum is foamed due to the decomposition of the TiH2 powder. The foamed aluminum obtained by this method includes pores having large pore diameters of several millimeters.
The following method is exemplified as a fifth method. As disclosed in Patent Document 6, aluminum is mixed with metal of which eutectic temperature with aluminum is lower than the melting point of aluminum, and the mixture is heated at a temperature which is higher than the eutectic temperature and lower than the melting point of aluminum. Foamed aluminum obtained by this method has a porosity of about 40% which is low, although the pore diameters can be reduced by this method. Therefore, in the case where the foamed aluminum is used as a current collector, an amount of cathode active material or anode active material infiltrated into the pores of the foamed aluminum is small, and the desired high output and high energy density cannot be achieved.
Accordingly, among the aforementioned foam-melting method and the second to fifth methods, the second method in which aluminum is pressed into a casting mold having a core of sponge urethane, is employed as a method of producing foamed aluminum including fine open pores which can attain the high output and high energy density.
However, even in the second method, it is necessary to use sponge urethane having a fine microporous structure in order to further reduce the pore diameters of the open pores. In the case where such a sponge urethane is used, the flow of aluminum worsens; and thereby, aluminum cannot be press-filled into the hollow, or the casting pressure becomes excessively high. Therefore, it is difficult to manufacture foamed aluminum which includes pores having pore diameters that fulfill smaller than 40 PPI.
A slurry foaming method is disclosed in Patent Document 7 as a method for producing foamed metal which has a high porosity and includes open pores having small diameters and uniform dimensions, in which a plurality of fine open pores are uniformly arranged. In the slurry foaming method, foamable slurry containing metal powder and a foaming agent is foamed, and dried. Thereafter, the foamed and dried slurry is sintered. According to this method, if a raw material powder which can be sintered is prepared, it is possible to easily manufacture foamed metal which has a high porosity and includes open pores having uniform dimensions and arbitrary pore diameters that fulfill about 10 PPI to about 500 PPI, that is, within a pore diameter range of 2.5 mm to 50 μm. Here, the slurry foaming method means a method for producing foamed metal in which foaming is conducted by containing the foaming agent as described above or foaming is conducted by injecting gas and stirring, and the foamable slurry as described above is sintered in the foamed state.
However, conventionally, it is difficult to manufacture foamed aluminum by the slurry foaming method.
The reason is as follows. In the slurry foaming method, metal powder is sintered by free sintering which is performed without applying stress such as a compression stress or the like; and thereby, foamed metal is obtained. However, the surface of aluminum powder is covered with dense aluminum oxide film having a thickness of several nanometers to several tens of nanometers, and this aluminum oxide film inhibits the sintering regardless of being solid phase sintering or a liquid phase sintering. Therefore, it is difficult to proceed sintering by the free sintering; and as a result, uniform foamed aluminum cannot be obtained by the slurry foaming method.
Therefore, a method can be exemplified which employs a combination of the slurry foaming method and the aforementioned fifth method, as a method for sintering the aluminum powder by the free sintering. According to this method, copper powder is prepared as a metal whose eutectic temperature with aluminum is lower than the melting point of aluminum, and the copper powder and a foaming agent are mixed with aluminum. Then the mixture is heated and sintered at a temperature which is higher than the eutectic temperature and lower than the melting point of aluminum. Thereby, foamed aluminum is obtained. However, liquid droplets of aluminum ooze out of the surface, and the liquid droplets are solidified; and as a result, a plurality of aluminum lumps having semispherical shapes are formed. In particular, in the case where the foamed aluminum has a thin plate shape, the formation of the aluminum lumps occurs remarkably as shown in FIG. 4, and it is not possible to manufacture desired uniform foamed aluminum.